Current matched all perovskite tandem solar cells with low lead perovskites achieving 31.9% efficiency and enhanced stability

Abstract

Multilayer tandem solar cells emerge as a transformative solution, leveraging multiple absorber layers with optimized bandgaps to capture and convert a broader spectrum of sunlight. This layered architecture overcomes the efficiency limitations of single-junction solar cells by minimizing transparency and thermalization losses while maximizing photon utilization across the solar spectrum. Although hybrid perovskites have demonstrated exceptional photovoltaic performance, their dependence on organic components often results in stability challenges under varying environmental conditions. To mitigate this issue, all-inorganic perovskites have emerged as a robust alternative, offering enhanced thermal and moisture stability along with reliable long-term performance. In the proposed design, a sustainable approach is adopted using a tin-based, low-lead, all-inorganic CsPb_0.75Sn_0.25IBr₂ (1.78 eV) for the top subcell absorber, paired with a lead-free double perovskite Cs₂TiI₆ (1.02 eV), in the bottom subcell, with the use of SCAPS − 1D simulator. Standalone analyses of the top and bottom subcells are conducted before tandem configuration implementation. Importantly, tandem design is optimized by investigating the current matching point by varying the absorber layer thicknesses (100–1000 nm). Illuminating the top subcell with the AM 1.5G spectrum and passing filtered light to the bottom subcell enables extensive light absorption and improved overall PCE. With a common current point at 16.83 mA/cm² the tandem design attains a peak PCE of 31.93%, accompanied by a fill factor (FF) of 86.84% and an open-circuit voltage (V_OC) of 2.18 V. These findings highlight the potential of this optimized tandem solar cell design to deliver high efficiency with enhanced stability, offering a promising pathway for sustainable and scalable photovoltaic technologies.

Introduction

Since decades, silicon-based solar cells have been the major stakeholder in the photovoltaic industry. However, the conversion efficiency of silicon-based solar cell designs is constrained by the theoretical Shockley-Queisser limit i.e., 33.7%¹. The thin film, low-cost perovskites are favoured over silicon due to their potential for high PCE (power conversion efficiency) and flexibility in the bandgap^2,3,4,5,6. The crystalline structure of perovskites is ABX₃, where A is organic/inorganic cation, B is divalent cation and X is a halide anion^7,8,9. With super exciting opto-electric features, first organic-inorganic perovskite solar cell was designed in the year 2009, with 3.8% of PCE¹⁰. In a relatively short period, the efficiency of hybrid perovskites has surged, with single-junction perovskite solar cells now reaching 26.7%¹¹. Hybrid (Organic-inorganic) perovskite materials are notably susceptible to environmental conditions such as temperature variation, atmospheric moisture and oxygen^12,13,14,15. This results in compositional and structural degradation of perovskites¹⁶. Temperature variation and moisture in the air lead to migrate organic cations in compounds, and hence disrupt the perovskite structure^17,18,19,20. Solar cell stability is an important aspect of photovoltaic performance that is considerable while designing a solar cell^21,22. Cesium based inorganic perovskite compounds have shown proven chemical and thermal stability compared to organic-inorganic perovskites^{23,24,25,26,27}. Niu et al., demonstrated improved stability in solar cell performance with cesium doped Cs_0.09MA_0.91PbI₃ perovskite compound and attained PCE of 18.1%²⁸. Later, Wang et al., proposed an all-inorganic CsPbI₃, hydroiodic acid and phenylethyl ammonium iodide additives that have improved the stability of distorted black phase²⁹. A bromide-doped all-inorganic CsPbI₂Br solar cell was designed by Liu et al. The device exhibited stable PV performance under thermal stress and, delivered 13.3% of PCE³⁰. Boro et al., varied the cesium composition in FA_1 − yCs_yPb(I_0.85Br_0.15)₃ (0 < y < 25) and observed that an optimal cesium content of 10% enhanced thermal stability. A FA_0.90Cs_0.10Pb(I_0.85Br_0.15)₃ absorber layer demonstrated a PCE of 17.7%³¹. Though lead-based perovskites have excellent optoelectronic properties, the presence of this metal in perovskite compounds is toxic to humankind^32,33,34. This concern has raised the alarm to explore other metals, in lead substitutes. Mixed halide perovskite compounds with reduced lead have shown good PV performance and also contributed to sustainable approach. Yang et al., has proposed an Sn - Pb alloy based all-inorganic perovskite solar cell, prepared by a facile solution process. The proposed strategy has overcome the limitations of Pb-based perovskites with an achieved PCE of 9.41% in an inverted structure³⁵. Liang et al., investigated the reduced lead all inorganic mixed halide CsPb_0.9Sn_0.1IBr₂, which possesses a band gap of 1.79 eV. With good long term stability, solar cell could deliver 11.33% of PCE³⁶. Similarly, Ban et al., designed a tin-based perovskite solar cell incorporating 1-(4-carboxyphenyl)-2-thiourea for surface coordination with Sn²⁺, achieving an efficiency of 8%. The device retained 90% of its initial conversion efficiency after 3000 h of storage in an N₂-filled glovebox³⁷. Hong et al. demonstrated the enhanced stability of CsPb_0.9Sn_0.1IBr₂-based perovskite solar cells compared to CsPbIBr₂. The incorporation of Sn²⁺ effectively minimized light-induced segregation³⁸. Additionally, Park et al., explored bromide composition adjustments in CsSn(I_1 − xBr_x)₃ through DFT analysis, demonstrating stable performance with a peak PCE of 11.87%³⁹.

Moreover, the use of lead-free double perovskites presents an environmentally sustainable alternative, addressing both stability and toxicity concerns associated with lead-based perovskites⁴⁰. Zhang et al.., reported a Cs₂AgBiBr₆-based perovskite solar cell with improved stability through hydrogen doping, achieved an efficiency of 6.37%⁴¹. Sen et al.., reported HTL-free Cs₂TiBr₆ double perovskite solar cell design, which attained a PCE of 2.34%⁴². Jaiswal et al., proposed Cs₂SnI₆ as a stable alternative, where the Sn²⁺ oxidation state enhanced its resistance to degradation⁴³. Additionally, Aslam et al.., investigated the structural and optical properties of Cs₂TiI₆, highlighted its strong absorption characteristics⁴⁴. Due to its suitable bandgap and stable performance, Cs₂TiI₆ double perovskite is an excellent candidate for the bottom subcell absorber, ensuring efficient low-energy photon absorption and enhanced overall device stability.

Importantly, multilayer tandem solar cell configuration (MTSC) demonstrates significantly higher PCE than traditional single-junction solar cells⁴⁵. The multilayered architecture facilitates the absorption of broader the light spectrum and minimized energy losses. The overall PV performance of solar cell enhanced, while optimizing photon absorption by two different subcells connected in series (electrically). The series connection in tandem configurations ensures efficient transfer of charge through subcells, maintaining the same current throughout. The top subcell is designed to absorb high-energy photons, whereas the bottom subcell is for low-energy photons. The optimal band gap range for top cell is 1.5 eV − 2.2 eV^46,47 while the bottom subcell, featuring perovskites with band gaps of 0.95 eV − 1.15 eV, efficiently captures lower-energy photons filtered through the top layer^48,49.

Despite the promising performance of perovskite-based solar cells, challenges such as lead toxicity and stability issues remain critical obstacles to commercialization. To address these concerns, we propose an MTSC design featuring a novel material combination that balances efficiency, stability, and environmental sustainability. A wide-bandgap low lead mixed halide perovskite CsPb_0.75Sn_0.25IBr₂ is used as the top absorber layer, while the highly stable, lead-free double perovskite Cs₂TiI₆, with a narrower bandgap, serves as the bottom absorber layer. The all-inorganic CsPb_1 − xSnIBr₂ (x – 0, 0.25, 0.5, 0.75, 1) series has shown excellent phase stability, and the composition CsPb_0.75Sn_0.25IBr₂, offered a band gap of 1.78 eV. This makes it an ideal candidate for the top subcell, as it efficiently absorbs high-energy photons⁵⁰. For the bottom subcell, Cs₂TiI₆, with its 1.02 eV band gap, effectively captures the lower-energy photons that pass through the top layer. This material not only provides excellent light absorption but also ensures stable photovoltaic performance, thus contributing to the overall efficiency, sustainability, and non-toxicity of the tandem solar cell structure^51,52. This novel approach promises a significant step forward in both performance and environmental impact.

Methodology

The proposed study presents a design of all inorganic MTSC, where the top subcell utilizes a wide bandgap absorber, and the bottom subcell employs a narrow bandgap material. The top subcell is constructed using the low-lead, all-inorganic perovskite CsPb_0.75Sn_0.25IBr₂ (as shown in Fig. 1(a)), while the bottom subcell features the lead-free double perovskite Cs₂TiI₆ (as shown in Fig. 1(b)). The individual performance of both subcells is first analyzed in a standalone configuration using SCAPS-1D (Solar Cell Capacitance Simulator – one-dimensional), a numerical simulation tool. The analysis is performed under AM 1.5 G light spectrum, one sun illumination of 1000 watts/m² at 300 K of temperature.

In tandem-configured solar cells, stacked wide band gap (CsPb_0.75Sn_0.25IBr₂) and narrow band gap (Cs₂TiI₆) absorbers, elevate the optical absorption and, consequently higher PCE (as shown in Fig. 1(c)). Before the tandem analysis, the performance of each standalone subcell is evaluated. Subsequently, the current matching technique is employed, to identify a common current that flows through both subcells, as the series connection of the subcells dictates that the lower current limits the overall efficiency of the tandem solar cell. For tandem analysis, the filtered light spectrum is calculated based on the absorption coefficient and thicknesses of the top subcell.

Due to the limitation on the number of layers in SCAPS-1D, the current matching technique is adopted to simulate a multi-layered tandem configuration. The compatible electron transport layer (ETL) and hole transport layer (HTL) for top cell are N-PDI and Spiro-OMeTAD (2,2’,7,7’-tetrakis[N, N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene) respectively. Further, for the bottom subcell TiO₂ (titanium dioxide) and PEDOT (Poly(3,4-ethylenedioxythiophene)) are used as ETL/HTL. The electrical properties of the various layers in the solar cell, as used in SCAPS-1D, are summarized in Table 1.

Table 1 Electrical parameters used for CsPb_0.75Sn_0.25IBr₂/Cs₂TiI₆ tandem solar cell, input to SCAPS − 1D simulator^{50,51,52,53,54,55,56,57,58,59,60}.

Fig. 1

Device structure of (a) standalone CsPb_0.75Sn_0.25IBr₂ absorber-based top cell (b) standalone Cs₂TiI₆ absorber-based bottom cell (c) CsPb_0.75Sn_0.25IBr₂/Cs₂TiI₆ based MTSC.

The incorporated absorption profiles for CsPb_0.75Sn_0.25IBr₂ and Cs₂TiI₆ are shown in Fig. 2 (a), (b) respectively. These profiles are generated using inbuilt SCAPS-1D optical models which is widely adopted by researchers^61,62.

Fig. 2

Absorption profiles for (a) CsPb_0.75Sn_0.25IBr₂ top sub cell absorber (b) Cs₂TiI₆ bottom sub cell absorber.

Results and discussions

This innovative approach presents a MTSC design that utilizes a tin-based perovskite absorber in the top subcell and a lead-free double perovskite in the bottom subcell. This study is structured in two key sections: the PV performance evaluation of the standalone subcells (top and bottom) and the analysis of the tandem configuration. Initially, the PV performance of both top and bottom standalone subcells is assessed using SCAPS-1D simulation software.

Top and bottom subcells under standalone condition

The proposed top cell features a layered architecture of front electrode/N-PDI/CsPb_0.75Sn_0.25IBr₂/Spiro-OMeTAD/back electrode, while the bottom cell is designed with a structure of front electrode/TiO₂/Cs₂TiI₆/PEDOT/back electrode. The simulation is conducted under standard light spectrum of AM 1.5G, in SCAPS-1D simulator⁶³. The current density (JV) and external quantum efficiency (EQE) plots for standalone top and bottom cells at 500 nm thick absorber layer are extracted and demonstrated in Fig. 3. By this approach, it is convenient to understand the optical performance of individual cell design and optimize the performance. Simulation results show a J_SC of 15.16 mA/cm² and a V_OC of 1.41 V for the standalone top cell, while the bottom subcell achieves a J_SC of 33.38 mA/cm² and a V_OC of 0.79 V. Under standalone conditions, the PCE of the top and bottom subcells are 18.77% and 22.63%, respectively. The EQE curve for the top and bottom subcells reached zero at 700 nm and 1200 nm light wavelengths, respectively.

Fig. 3

Standalone top subcell (front electrode/N-PDI/CsPb_0.75Sn_0.25IBr₂/Spiro-OMeTAD/back electrode, ) (a) JV plot (b) EQE plot, bottom subcell (front electrode/TiO₂/Cs₂TiI₆/PEDOT/back electrode) (c) JV plot (d) EQE plot.

Further analyses were conducted on the top and bottom subcells under biased (V_OC) and unbiased (no V_OC) conditions. Figure 4(a), presents the energy band diagram (EBD), highlighting the flattened band structure under biased conditions (V_OC). Additionally, Fig. 4(b-c) illustrates the distribution of electron and hole carrier densities as well as the corresponding electron and hole current densities. Figure 4(b), demonstrate the electron/hole distribution across the solar cell, with the ETL region exhibiting a higher electron concentration, the HTL region showing a higher hole concentration, and the absorber layer in between displaying an equal concentration of both.

Fig. 4

Standalone top subcell (a) Energy band diagram (eV) (b) Carrier density (1/cm³) (c) Current density (mA/cm²), under biased voltage (V_OC).

Further, standalone top cell is simulated under unbiased condition (no V_OC applied) and corresponding EBD is shown in Fig. 5 (a). The carrier and current density under no bias condition is well demonstrated in Fig. 5 (b–c). For the bottom cell, the same analysis has been performed under biased conditions and represented in terms of EBD, carrier density, and current density plots, as depicted in Fig. 6 (a–c). The carrier density plot gives information about the electron and hole charge carrier distribution along the cell depth. The current density plot gives insight into electron/hole carrier transport, and how electron/hole current flows which ultimately contributes to PCE. Alternatively, EBD, electron/hole carrier density, and current density of standalone subcell under no bias condition, are shown in Fig. 7(a–c).

Fig. 5

Standalone top subcell (a) Energy band diagram (eV) (b) Carrier density (1/cm³) (c) Current density (mA/cm²), under unbiased condition (0 V).

Fig. 6

Standalone bottom subcell (a) Energy band diagram (eV) (b) Carrier density (1/cm^− 3) (c) Current density (mA/cm²), under biased voltage (V_OC).

Fig. 7

Standalone bottom subcell (a) Energy band diagram (eV) (b) Carrier density (1/cm³) (c) Current density (mA/cm²), under unbiased condition (0 V ).

Impact of ETL/HTL thickness variation

This section of the manuscript presents a standalone analysis that examines the effects of varying the thickness of the ETL and HTL layers in both the top and bottom subcells. In the proposed MTSC configuration, N-PDI serves as the ETL and Spiro-OMeTAD as the HTL for the top subcell. The thickness of the ETL and HTL layers in both subcells is varied within the range of 50 nm to 250 nm, and their effects on PV performance are analysed. It has been a clear observation from Fig. 8(a), that an increase in N-PDI thickness heightened recombination rates, resulting in a decrease in J_SC from 15.17 mA/cm² to 12.20 mA/cm². However, V_OC shows the insignificant decline from 1.41 V to 1.40 V. Furthermore Fig. 8(b), demonstrates the decrease in PCE from 18.77% to 14.96, with an increase in ETL thickness. Moreover, the FF is varied over the range of 87.65–87.31% for ETL thickness (50 to 250 nm).

Fig. 8

Standalone top subcell on ETL thickness variation (50–250 nm) (a) IV curve (b) PV parameters, HTL thickness variation (c) IV curve (d) PV parameters.

In contrast, variations in the thickness of HTL have minimal impact on PV performance because limited number of photons reaching the HTL of the solar cell. This limitation arises from the absorption properties of the active layer, which primarily captures incoming light. As a result, changes in Spiro-OMeTAD thickness do not significantly influence charge transport or collection efficiency, since the key processes occur predominantly within the active layer. It can be verified from Fig. 8 (c), that variation in Spiro-OMeTAD thickness has not influenced the carrier generation, hence J_SC remains unaltered. Specifically, J_SC is maintained at 15.17 mA/cm² across the range of HTL thicknesses from 50 nm to 250 nm. The other PV parameters, including V_OC, FF, and PCE, are recorded at 1.41 V, 87.41%, and 18.72%, respectively, at a Spiro-OMeTAD thickness of 250 nm. However, negligible variation is noted in PV parameters w.r.t Spiro-OMeTAD thickness, as seen in Fig. 8 (d).

Similar to top subcell, the PV performance of bottom subcell is optimized over the ETL thickness range 50 –250 nm. The results indicate that there are only minor variations in the PV parameters with changes in the TiO₂ (ETL) thickness. As shown in Fig. 9 (a), the J_SC exhibits negligible variation, decreasing from 33.38 mA/cm² to 33.15 mA/cm² as TiO₂ thickness increases. Meanwhile, the V_OC and FF remain stable at 0.79 V and 85.80%, respectively, throughout the entire TiO₂ thickness range. Furthermore, Fig. 9 (b) indicates that the highest PCE of 22.63% is achieved at a TiO₂ thickness of 250 nm.

Fig. 9

Standalone bottom subcell on ETL thickness variation (a) IV curve (b) PV parameters, HTL thickness variation (c) IV curve (d) PV parameters.

HTL thickness has not influenced the PV performance of the bottom subcell, as shown in Fig. 9 (c - d). The fixed value of J_SC, V_OC, FF, and PCE remains unchanged at 33.38 mA/cm², 0.79 V, 85.80% and 22.63%, throughout the variations in PEDOT thickness.

Impact of interface defects on top/bottom subcells

In this section, interface defects are examined to assess their impact on the overall performance of the standalone top and bottom subcells. Initially, the interface defect at N-PDI/CsPb_0.75 Sn_0.25IBr₂ (ETL/absorber layer) layer of top subcells is varied over 10¹⁰ cm^− 2 to 10¹⁸ cm^− 2 and their PV performance is demonstrated in Fig. 10 (a - b). An increase in interface defects leads to higher recombination rates, which subsequently degrade solar cell performance. A decline is noticed in J_SC, when defects are increased from 10¹⁰ cm^− 2 to 10¹⁸ cm^− 2. A similar trend has been seen in Fig. 10 (b), for V_OC, FF, and PCE. The highest obtained from top subcell is 18.77% at 10¹⁰ cm^− 2 and, it declines to 14.13% at 10¹⁸ cm^− 2 of N-PDI/CsPb_0.75Sn_0.25IBr₂ interface defect. In contrast, defect variations at the CsPb_0.75Sn_0.25IBr₂/Spiro-OMeTAD interface do not impact carrier generation, as only a limited portion of light wavelengths reaches the absorber/HTL interface. It can be well observed from Fig. 10 (c), interface defect adjustment from 10¹⁰ cm^− 2 to 10¹⁸ cm^− 2, shows a minor variation in J_SC from 15.17 mA/cm² to 15.10 mA/cm². However, the declination observed in V_OC is significant, from 1.41 V to 1.28 V with increased interface defects. Figure 10 (d), illustrated a drop in PCE from 18.77 to 16.95% with a rise in interface defects from 10¹⁰ cm^− 2 to 10¹⁸ cm^− 2.

Fig. 10

Impact of interface defects (10¹⁰ cm^− 2 to 10¹⁸ cm^− 2) on the performance of the top subcell (a) IV curve and (b) PV parameters for N-PDI/CsPb_0.75 Sn_0.25IBr₂ (ETL/absorber layer) interface (c) IV curve and (d) PV parameters for CsPb_0.75 Sn_0.25IBr₂/Spiro-OMeTAD (absorber layer/HTL) interface.

Further investigation is carried out to evaluate the performance of the bottom subcell under the influence of interface defects. This includes assessing how defects at the ETL/absorber layer (TiO₂/Cs₂TiI₆) and absorber layer/HTL (Cs₂TiI₆/PEDOT) interfaces affect overall PV performance. The analysis of the bottom subcell is performed across a defect range of 10¹⁰ cm^− 2 to 10¹³ cm^− 2, varied in 8 logarithmic steps. Notably, defects at the TiO₂/Cs₂TiI₆ interface significantly impact the J_SC, which decreases sharply from 33.38 mA/cm² to 10.61 mA/cm² as the defect density increases from 10¹⁰ cm^− 2 to 10¹³ cm^− 2, seen in Fig. 11 (a). In contrast, V_OC and FF exhibit only minor fluctuations across this defect range. Figure 11 (b), also illustrates a substantial decline in PCE for the bottom subcell, dropping from 22.63 to 6.66% as interface defect densities increase.

In contrast, defects at the Cs₂TiI₆/PEDOT interface show no effect on the J_SC, which remains steady at 33.38 mA/cm², shown in Fig. 11- (c). However, Fig. 11 (d), demonstrates a slight variation in the V_OC and FF, decreasing from 0.79 V to 0.75 V and from 85.80 to 83.58%, respectively, with increasing interface defects. Consequently, PCE experiences a modest decline, dropping from 22.63 to 20.87%.

Fig. 11

Impact of interface defects ((10¹⁰ cm^− 2 to 10¹³cm⁻²) on the performance of the bottom subcell (a) IV curve and (b) PV parameters for TiO₂/Cs₂TiI₆ ( ETL/absorber layer) interface (c) IV curve and (d) PV parameters for Cs₂TiI₆/PEDOT (absorber layer/HTL) interface.

Impact of standalone top/bottom subcell absorber layer thickness variation

In this section of the manuscript, the thickness of absorber layers (standalone top/bottom subcell) is adjusted over the range 100–1000 nm, under light spectrum AM 1.5G, to optimize the PV performance of solar cells. Initially, for the top cell, an increase of CsPb_0.75Sn_0.25IBr₂ from 100 to 1000 nm, enhances light absorption and boosts light-generated carriers. These results increase in J_SC, and hence PCE as shown in Fig. 12 (a). The highest recorded value of PCE is 20.66% at 1000 nm of CsPb_0.75Sn_0.25IBr₂. Additionally, the FF shows an improvement from 84.24 to 87.63%, while the V_OC shows a slight decrease from 1.41 V to 1.40 V with increasing thickness.

For the bottom subcell, the highest recorded PCE is 24.55% at a Cs₂TiI₆ thickness of 300 nm. As shown in Fig. 12 (b) increasing the thickness initially boosts J_SC by improving photon absorption, leading to a corresponding rise in PCE. Beyond 300 nm, however, PCE begins to decline due to increased internal recombination. A slight variation is observed in the V_OC, ranging from 0.81 V to 0.77 V, while the FF remains relatively stable between 85.31% and 85.80%.

Fig. 12

Effect of thickness variation (100–1000 nm) on PV parameters for (a) the top subcell absorber layer, CsPb_0.75Sn_0.25IBr_2, and (b) the bottom subcell absorber layer, Cs₂TiI₆.

Tandem solar cell simulation

The tandem configuration of a CsPb_0.75Sn_0.25IBr₂/Cs₂TiI₆ solar cell is simulated using SCAPS-1D, with CsPb_0.75Sn_0.25IBr₂ as the top-cell absorber (wide bandgap) and Cs₂TiI₆ as the bottom-cell absorber (narrow bandgap). Due to the limitation of SCAPS − 1D in the design of multilayered structures, each subcell is designed separately. By adjusting the thicknesses of the subcells, a common current point is determined, which dictates the maximum current through the tandem cell and is crucial for optimizing optical coupling. In this configuration, the top subcell absorbs the full AM1.5G light spectrum, while the filtered spectrum passes to the bottom cell, enhancing overall light absorption and energy conversion across the device. In a tandem solar cell, the top, and bottom subcells are electrically connected in series, consequently, the lowest J_SC between the subcells limits the overall current of the tandem cell. Meanwhile, the V_OC of the tandem cell is the sum of the V_OC values of each subcell. Figure 13 (a, b), illustrates both the AM 1.5G spectrum and the transmitted filtered spectrum from the top cell.

$$T(\lambda ) = T_{o} (\lambda ) \cdot \exp \left( {\sum\limits_{{i = 1}}^{3} { - (\alpha _{{material_{i} }} (\lambda ) \cdot t_{{material_{i} }} )} } \right)$$

(1)

Whereas T₀(λ) = incident AM1.5G light spectrum.

α is the absorption coefficient of materials, material₁ - N-PDI, material₂ - CsPb_0.75Sn_0.25IBr₂, and material₃ - Spiro-MeOTAD and t represent the thickness of material.

α is the absorption coefficient of materials, material₁ - N-PDI, material₂ - CsPb_0.75Sn_0.25IBr₂, and material₃ - Spiro-MeOTAD and t represent the thickness of respective material.

To achieve optimal current matching in the tandem cell configuration, the thickness of the top cell is systematically varied from 100 nm to 1000 nm across ten incremental steps. The top cell is illuminated with standard light spectrum AM 1.5G and transmitted spectrum T(λ) to bottom subcell is calculated. At each thickness (material₂) of top absorber layer, the transmitted filtered light spectrum is determined using Eq. (1), considering the material absorption coefficient.

The ten computed filtered spectra are subsequently applied to the bottom subcell, whose thickness is also varied within the 100 –1000 nm range to determine the current matching points. This is achieved by simulating the J_SC of the bottom subcell under each filtered spectrum and then plotting the J_SC values of both the top and bottom subcells together, as illustrated in Fig. 2, to identify their intersection, which corresponds to the current matching point.

Fig. 13

(a) The standard light spectrum AM 1.5 G (b) The filtered light spectrum at CsPb_0.75Sn_0.25IBr₂ thickness variation (100–1000 nm).

Fig. 14

J_SC (short circuit current density) plot at 100–1000 nm thickness variation of CsPb_0.75Sn_0.25IBr₂ (top cell absorber) and Cs₂TiI₆ (bottom cell absorber), illustrating the evaluation of the current matching point.

Over the analysis, the current matching point is evaluated at 970 nm of CsPb_0.75Sn_0.25IBr₂ top cell absorber thickness and 200 nm of Cs₂TiI₆ of bottom cell absorber thickness. The evaluated J_SC at the common current point is 16.84 mA/cm², shown in Fig. 14.

Apart from the current matching point, further analysis is conducted to thoroughly evaluate the PV performance of tandem-configured solar cell. It has observed from Fig. 15 (a), that increasing the thickness of the absorber layers in both the top and bottom cells, from 100 to 1000 nm, resulted in a gradual decline in the V_OC. However, the highest V_OC achieved is 0.8 V, with absorber layer thicknesses of 100 nm for both CsPb_0.75Sn_0.25IBr₂ in the top cell and Cs₂TiI₆ in the bottom cell. Furthermore, Fig. 15 (b) shows that this configuration resulted in the highest FF, achieving 85.78%. Additionally, the peak J_SC has been recorded at 28.36 mA/cm² when the CsPb_0.75Sn_0.25IBr₂ layer in the top cell has been set to 100 nm and the Cs₂TiI₆ layer in the bottom cell to 300 nm, shown in Fig. 15 (c). Based on these findings, the analysis concluded with the highest PCE of 24.6%, achieved with this optimal thickness combination of 100 nm for the top and 300 nm for the bottom absorber layers, as depicted in Fig. 15 (d). Further, Fig. 16 (a-b), presents a comparative analysis of the JV (current density-voltage) plot for standalone and tandem configurations, analysed under the AM 1.5G spectrum for the standalone case and filtered spectrum for the tandem configuration. In the tandem configuration, the overall J_SC is limited by the lower J_SC of the top subcell due to the series connection, resulting in a tandem J_SC of 16.84 mA/cm², which represents the current matching point (where the same current flows through both subcells). The V_OC of MTSC reaches 2.18 V, as it is the sum of the V_OC values from the top (1.39 V) and bottom (0.79 V) subcells. Notably, this low-lead, all-inorganic MTSC achieves a PCE of 31.93%, as mentioned in Table 2.

Fig. 15

Obtained PV parameters (a) V_OC (b) FF (c) J_SC (d) PCE, representing the cumulative impact of top and bottom cell thickness variation (100–1000 nm ), under filtered spectrum.

Fig. 16

JV plots in regards to (a) standalone top/bottom subcell (under AM 1.5 G spectrum) (b) MTSC CsPb_0.75Sn_0.25IBr₂/Cs₂Ti₆ under filtered spectrum.

Table 2 PV performance of top, bottom subcell (under filtered spectrum) and tandem cell at the current matching point.

Furthermore, the photovoltaic performance of this study is compared with previously published works, as summarized in Table 3. This work presents a novel all-inorganic tandem solar cell, integrating a low-lead CsPb_0.75Sn_0.25IBr₂ top subcell with a lead-free Cs₂TiI₆ bottom subcell, enhancing stability and efficiency. By optimizing absorber thicknesses for current matching using SCAPS-1D simulations, the design achieves a high PCE of 31.93%, offering a promising pathway for sustainable photovoltaic technologies.

Table 3 Comparative performance analysis of all perovskite tandem solar cell designs.

Conclusion

In conclusion, this study marks a significant breakthrough in developing environment-friendly MTSC design with promising PCE. The top subcell utilizes the low-lead, tin-based CsPb_0.75Sn_0.25IBr₂ as the primary absorber material, while the lead-free double perovskite Cs₂Ti₆ serves as the bottom subcell, featuring respective bandgaps of 1.78 eV (wide band gap) and 1.02 eV (narrow band gap). Individually, the top and bottom subcells achieved PCEs of 18.77% and 22.63%, respectively. For MTSC, the top subcell is exposed to the AM 1.5G standard light spectrum, while the bottom subcell receives filtered light. By adjusting absorber layer thicknesses, an optimal current matching point of 16.84 mA/cm² has been identified at 970 nm and 200 nm for the top and bottom absorber layers, respectively. This alignment enhances light coupling across the subcells, resulting in a peak PCE of 31.9% and a V_OC of 2.18 V in the tandem design. Future research may focus on improving material stability and refining design for better current matching and scalability, paving the way for efficient, sustainable, and commercially viable tandem solar cell technologies.